Heat pipe with composite wick structure

ABSTRACT

An exemplary heat pipe includes an elongated casing, a first wick structure, a second wick structure, and working medium filled in the casing. The heat pipe has an evaporating section and a condensing section. The first wick structure is located within an inner wall of the casing and defines a window at the evaporating section of the heat pipe. The first wick structure has a first pore size. The second wick structure is received in the window of the first wick structure. The second wick structure is in direct physical contact with the inner wall of the evaporating section of the casing and the first wick structure. The second wick structure has a second pore size smaller than the first pore size of the first wick structure. The working medium saturates the first wick structure and the second wick structure.

BACKGROUND

1. Technical Field

The disclosure relates generally to a heat transfer apparatus, and more particularly to a heat pipe having a composite capillary wick structure.

2. Description of the Related Art

Heat pipes have excellent heat transfer performance due to their low thermal resistance, and are therefore an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers.

A heat pipe is usually a vacuum casing containing a working medium therein. The working medium is employed to carry, under phase change between liquid state and vapor state, thermal energy from an evaporator section to a condenser section of the heat pipe. Preferably, a wick structure is provided inside the heat pipe, attached to an inner wall of the casing, for drawing the working medium back to the evaporator section after it is condensed at the condenser section. In operation, the evaporator section of the heat pipe is maintained in thermal contact with a heat-generating component. The working medium contained at the evaporator section absorbs heat generated by the heat-generating component and then turns into vapor and moves towards the condenser section where the vapor is condensed into condensate after releasing the heat into the ambient environment. Due to the difference in capillary pressure which develops in the wick structure between the two sections, the condensate is then brought back by the wick structure to the evaporator section where it is again available for evaporation.

In order to draw the condensate back timely, the wick structure provided in the heat pipe is expected to provide a high capillary force and meanwhile generate a low flow resistance for the condensate. In ordinary use, the heat pipe needs to be flattened to enable the miniaturization of electronic products incorporating the heat pipe. The flattening may result in damage to the wick structure of the heat pipe. When this happens, the flow resistance of the wick structure increases and the capillary force provided by the wick structure is decreased, which in turn reduces the heat transfer capability of the heat pipe. If the condensate is not quickly brought back from the condenser section, the heat pipe will suffer a dry-out problem at the evaporator section.

Therefore, it is desirable to provide a heat pipe with improved heat transfer capability; wherein a wick structure of the heat pipe will not be damaged and still can have a satisfactory wicking force when the heat pipe is flattened.

BRIEF DESCRIPTION OF THE DRAWINGS

The components of the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments of the display device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout several views.

FIG. 1 is a longitudinally cross-sectional view of a heat pipe in accordance with a first embodiment of the present disclosure.

FIG. 2 is a transversely cross-sectional view of an evaporating section of the heat pipe of FIG. 1.

FIG. 3 is a top view of first wick structure of the heat pipe of FIG. 1 before being coiled.

FIG. 4 is a longitudinally cross-sectional view of a heat pipe in accordance with a second embodiment of the present disclosure.

DETAILED DESCRIPTION

Referring to FIG. 1, a heat pipe 100 in accordance with a first embodiment of the disclosure is shown. The heat pipe 100 is a plate-type heat pipe, and includes a flat tube-like metal casing 10 with two ends thereof being sealed, a first wick structure 30 and a second wick structure 40 enclosed in the metal casing 10, and a working medium 20.

Referring to FIGS. 2 and 3, the casing 10 is made of high thermally conductive material such as copper or aluminum. A width of the casing 10 is larger than a height of the casing 10. To accommodate lightweight requirements of electronic products, the height of the casing 10 is preferably not larger than 2 mm. The heat pipe 100 has an evaporating section 102, an opposing condensing section 104 along a longitudinal direction of the heat pipe 100, and an adiabatic section 103 located between the evaporating section 102 and the condensing section 104. The working medium 20 is saturated in the first wick structure 30 and the second wick structure 40. The working medium 20 is usually selected from a liquid, such as water, methanol, or alcohol, which has a low boiling point. The casing 10 of the heat pipe 100 is evacuated and hermetically sealed after the working medium 20 is injected into the casing 10 and saturated in the first wick structure 30 and the second wick structure 40. Thus, the working medium 20 can easily evaporate to vapor when it receives heat at the evaporating section 102 of the heat pipe 100.

The first wick structure 30 is screen mesh to provide a capillary force to drive condensed working medium 20 at the condensing section 104 to flow towards the evaporating section 102; thus, a thickness and pore size of the first wick structure 30 can be easily changed. The thickness of the first wick structure 30 is preferably smaller than 0.1 mm. The first wick structure 30 is attached on an inner wall of the casing 10 and extends from the condensing section 104 to the evaporating section 102. The first wick structure 30 is coiled from a plat screen mesh 31 showed as FIG. 3. The plat screen mesh 31 has a rectangular shape and defines a window 32 in an end thereof. The window 32 is defined corresponding to a portion of the evaporating section 102 opposite to a heat source (not shown). In this embodiment, the window 32 is rectangular and has a size as same as that of the heat source. The second wick structure 40 is sintered powder wick structure. The second wick structure 40 is received in the window 32 and attached to the inner wall of the casing 10. The second wick structure 40 connects and contacts the first wick structure 30 in the window 32. The second wick structure 40 has a pore size smaller than that of the first wick structure 30 to provide a larger capillary force than the first wick structure 30.

When assembled, the first wick structure 30 and the second wick structure 40 are juxtaposed on the inner wall of the casing 10. The second wick structure 40 is attached at a bottom surface of the inner wall of the evaporating section 102 of the case 10. A composite wick structure is thus formed in the casing 10 of the heat pipe 100. In operation, the evaporating section 102 of the heat pipe 100 is placed in thermal contact with the heat source. The working medium 20 contained in the evaporating section 102 of the heat pipe 100 is vaporized into vapor upon receiving the heat generated by the heat source. Then, the generated vapor moves via the inner space between the first wick structure 30. After the vapor releases the heat carried thereby and is condensed into condensate in the condensing section 104, the condensate flows through the pores of the first wick structure 30 to the evaporating section 102 of the heat pipe 100 to again be available for evaporation. Meanwhile, the condensate is capable of entering the second wick structure 40 easily due to the second wick structure 40 has smaller pores with the larger capillary force than the first wick structure 30. As a result, the condensate is drawn back to the evaporating section 102 rapidly and timely, thus preventing a potential dry-out problem occurring at the evaporating section 102 of the heat pipe 100. The composite wick structure has different pore sizes to provide relatively large capillary force, and to provide relatively low flow resistance and heat resistance at the same time. The heat transfer capability of the heat pipe 100 is thus increased.

Referring to FIG. 4, a heat pipe 100 a in accordance with a second embodiment includes a flat tube-like metal casing 10 with two ends thereof being sealed, a first wick structure 30 a and a second wick structure 40 enclosed in the metal casing 10, and a working medium 20. The heat pipe 100 a has an evaporating section 102, an opposing condensing section 104, and an adiabatic section 103. The first wick structure 30 a defines a window 32 in the evaporating section 102 to receive the second wick structure 40. The difference of the heat pipe 100 a from the heat pipe 100 of the first embodiment is that the first wick structure 30 a defines a plurality of additional windows 32 a to provide lower flow resistance. There is no second wick structure 40 in the additional windows 32 a.

It is to be further understood that even though numerous characteristics and advantages have been set forth in the foregoing description of the embodiment(s), together with details of the structures and functions of the embodiment(s), the disclosure is illustrative only; and that changes may be made in detail, especially in the matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A heat pipe, comprising: an elongated casing having an evaporating section and a condensing section; a first wick structure located within an inner wall of the casing, the first wick structure defining a window at the evaporating section of the heat pipe, the first wick structure having a first pore size; a second wick structure received in the window of the first wick structure, the second wick structure being in direct physical contact with the inner wall of the evaporating section of the casing and the first wick structure, the second wick structure having a second pore size smaller than the first pore size of the first wick structure; and working medium filled in the casing and saturating the first wick structure and the second wick structure.
 2. The heat pipe of claim 1, wherein the first wick structure is screen mesh and the second structure is sintered powder wick structure.
 3. The heat pipe of claim 1, wherein the heat pipe is a flat-type heat pipe, a width of the casing being larger than a height of the casing.
 4. The heat pipe of claim 3, wherein the height of the casing is not larger than 2 mm.
 5. The heat pipe of claim 1, wherein the first wick structure defines at least one additional window.
 6. The heat pipe of claim 1, wherein the window of the first wick structure is rectangular.
 7. The heat pipe of claim 1, wherein an area of the second wick structure is the same as an area of the window of the first wick structure.
 8. A heat pipe, comprising: an elongated casing having an evaporating section and a condensing section; a composite wick structure comprising a screen mesh wick structure and a sintered powder wick structure located at an inner wall of the casing, the screen mesh defining a window at the evaporating section of the heat pipe, the sintered powder wick structure being received in the window of the screen mesh wick structure, the sintered powder wick structure being in direct physical contact with the inner wall of the evaporating section of the casing and the screen mesh wick structure, the sintered powder wick structure having a pore size smaller than a pore size of the screen mesh wick structure; and working medium filled in the casing and saturating the screen mesh wick structure and the sintered powder wick structure.
 9. The heat pipe of claim 8, wherein the heat pipe is a flat-type heat pipe, a width of the casing being larger than a height of the casing.
 10. The heat pipe of claim 9, wherein the height of the casing is not larger than 2 mm.
 11. The heat pipe of claim 8, wherein the screen mesh wick structure defines at least one additional window.
 12. The heat pipe of claim 8, wherein the window of the screen mesh wick structure is rectangular.
 13. The heat pipe of claim 8, wherein an area of the sintered powder wick structure is the same as an area of the window of the screen mesh wick structure. 